אֶת־יְהוָ֧ה אֱלֹהֶ֛יךָ תִּירָ֖א אֹת֣וֹ תַעֲבֹ֑ד וּב֣וֹ תִדְבָּ֔ק וּבִשְׁמ֖וֹ תִּשָּׁבֵֽעַ׃
C’est l’Éternel, ton Dieu, que tu dois révérer, c’est lui que tu dois servir; attache-toi à lui seul, ne jure que par son nom.
Sefer HaMitzvot
The 5th mitzvah is that we are commanded to serve G‑d (blessed be He). This commandment is repeated twice. It is stated:"And you shall serve G‑d, your L‑rd"(Exodus 23:25). And it is also stated: "And you shall serve Him"(Deuteronomy 13:5). Although this commandment is of a general nature, as explained in the Fourth Principle, [and apparently should not be included in the count of the 613 mitzvos,] nevertheless it has a specific quality, since it is the commandment to pray. [We see that "service" is not just a general command from the following statements:] The Sifri says, "The verse, 'And to serve Him' means prayer." The Sages also said, "The verse, 'And to serve Him' means Torah study. In the Mishnah of Rabbi Eliezer, the son of Rabbi Yosi HaG'lili, the Sages said, "What is the biblical source to include prayer among the mitzvos? From the verse, 'You shall fear G‑d, your L‑rd, and you shall serve Him.' " They also said, "Serve Him through His Torah; serve Him in His Temple." This [statement, 'serve Him in His Temple,'] means that one's goal should be to pray in the Temple or in the direction of the Temple, as King Solomon, peace be upon him, explained.
Rashi
את ה׳ אלוהיך תירא THOU SHALT FEAR THE LORD THY GOD, and serve Him and cleave to Him; and after you possess all these qualities, then you may swear by His Name (cf. Rashi on 6:13 and Note thereon).
Chizkuni
את ה' אלוקיך תירא, “You shall fear the Lord your G-d;” This is a warning not to transgress negative commandments.